The invention relates to a process for removal of hydrogen sulphide and ammonia from gaseous streams comprising these compounds by combustion of ammonia into nitrogen and water vapor and partial combustion of hydrogen sulphide into sulphur dioxide, whereby hydrogen sulphide reacts with the sulphur dioxide formed in a molar ratio of 2:1 under formation of elementary sulphur and water.
A partial combustion of hydrogen sulphide to sulphur dioxide and combination of sulphur dioxide with the double molar amount of hydrogen sulphide, forming elementary sulphur and water, is generally known as the Claus process.
This commonly known process is generally applied at present in oil refineries, where the process plays an important role in the conversion of gases originating from the removal of sulphur compounds from crude petroleum distillates.
In the processing of crude oil gaseous products originate and besides desired products like propane, butane, gasoline, kerosene and combustion oil a residue remains. In the sulphur removal from these products the sulphur present therein is largely converted into hydrogen sulphide by the hydrogen gas added.
The hydrogen sulphide containing gases obtained from these sources are separated from the still present hydrocarbons in a gas purification plant (mostly an "amine treater"). The hydrogen sulphide rich gas issuing from this "amine treater" cannot be directly blown off into the atmosphere; therefore it is converted in a sulphur recovery plant into elementary sulphur and combustion products.
The combustion products may be vented through the chimney.
Since several refinery gases and liquids are contacted with water and/or steam during their processing, another waste product of the oil refinery is the polluted water, commonly referred to as "sour water". This polluted water comprises amongst others ammonia and hydrogen sulphide in dissolved condition and therefore it may neither be directly dispensed with. In the stripping of this sour water with steam a mixture of gases is obtained, in which usually ammonia and hydrogen sulphide are present in substantially equal volumetric quantities.
Till recently this mixture of gases was directly combusted in an afterburn furnace or in a torch, since a conventional sulphur recovery plant is not suitable for processing ammoniacal gas. The presence of ammonia may give rise to cloggings, because ammonia may combine with acidic compounds like H.sub.2 S, SO.sub.2, SO.sub.3 and nitrogen oxides forming salts which may deposit as solids.
Because of its hydrogen sulphide content a search was made for processes to use this gas nevertheless in a sulphur recovery plant according to the Claus process. It was found that the removal of ammonia is obtained by combustion of the mixture of hydrogen sulphide gas and ammonia gas in a sulphur recovery plant according to the Claus process with sufficient air in order to burn up the ammonia gas substantially completely to nitrogen gas and water vapor.
The hydrogen sulphide gas from the amine treater and the sour water stripper is partially combusted to sulphur dioxide. After combustion the molar ratio of hydrogen sulphide to sulphur dioxide should be 2:1.
The sulphur dioxide formed reacts with still present hydrogen sulphide according to the equilibrium reaction ##EQU1## Hydrocarbons possibly present in the gas are combusted in the flame into water vapor and carbon dioxide gas.
The combustion may take place in two ways: Firstly by total combustion of the gas from the sour water stripper. Secondly by mixing the gas from the sour water stripper with the gas from the amine treater and subsequent simultaneous combustion.
When the gas from the sour water stripper is separately completely combusted, there arises: either an insufficient amount of sulphur dioxide because of binding with the hydrogen sulphide gas coming from the amine treater under formation of sulphur and water, so that a part of the hydrogen sulphide gas from the amine treater should still be combusted; or a too large amount of sulphur dioxide, in which case the desired ratio of hydrogen sulphide to sulphur dioxide of 2:1 cannot be reached.
If in this first case there is formed such a large proportion of sulphur dioxide that the desired ratio of 2:1 will be approximated, then only a small proportion of the hydrogen sulphide from the amine treater should be combusted, which will make the flame unstable.
Another drawback of complete combustion of the gas from the "sour water stripper" is that because of the high combustion temperature and the oxidizing environment a continuing oxidation will take place besides the combustion into nitrogen gas and water vapor, giving rise to the formation of nitrogen oxides. Also a partial further oxidation of sulphur dioxide into sulphur trioxide takes place. Because besides such combustion products there always remains some unburnt ammonia gas in the combustion products, corresponding ammonium salts may be formed, like ammonium sulphate, ammonium sulphite and possibly ammonium nitrate and ammonium nitrite. These salts give rise to cloggings.
Besides the above-mentioned problems also a fast corrosion of the burner has been established in the complete combustion of the gas from the sour water stripper, caused by the high combustion temperature.